PEOSPECT EIDGE SECTION. 103 



frequent changes in the physical conditions of the alternating beds of shale 

 and limestone and partly to the fact that this series of beds forms the sum- 

 mit of the ridge and, lying nearer the axis of the fold, has been subjected 

 to much greater pressure and strain. The shales, yielding easily to pressure, 

 have folded and flexed under excessive strain, while the more compact 

 limestones, under the same force, were faulted and fissured. Evidence of 

 this is seen in the Mountain shale belt and the overlying limestone, the 

 former exhibiting a tendency to flatten out and the latter to recover the 

 normal dip by a sharp break, causing numerous fissures and faults. Since 

 the first uplifting of the mountain, intrusive dikes of rhyolite have filled 

 preexisting fissures and broadened lines of weakness, besides causing addi- 

 tional faulting and displacement. These intrusive masses, however, are 

 for the most part narrow and have produced no fundamental structural 

 changes, but much of the secondary alterations, such as local meta- 

 morphism of beds, the cementation of brecciated limestone, and similar 

 phenomena, are easily explained by their action. 



Numerous tunnels, run for the purpose of mining exploration, vary- 

 ing from 50 feet to several hundred feet in length, penetrate the Prospect 

 Mountain limestone all along the ridge, at different elevations. Among 

 them may be mentioned the Fourth of July, Maryland, Lemon, and 

 Golden Era tunnels. Most of them, however, extend only for short dis- 

 tances, and, while they offer fair sections of portions of the great limestone 

 belt and may have subserved the purposes of the miner, are of but little 

 value for purposes of geological structure. Two tunnels, the Eureka and 

 Prospect Mountain, running at right angles to the strike of the beds and 

 from opposite sides of the ridge, give admirable sections across nearly the 

 entire thickness of the limestone belt. 



Eureka Tunnel. The entrance to the Eureka tunnel is situated near the 

 head of Goodwin Canyon, to the west of the Hamburg Ridge (atlas sheet 

 vm). The tunnel starts in near the base of the Hamburg limestone and 

 is driven in a nearly due west direction for 2,000 feet, passing several 

 hundred feet beyond the crest of the ridge and about 800 feet below. 

 The following is the series of beds encountered in the Eureka tunnel, 

 beginning at the entrance: 



